Land mobile radio systems provide for two way radio communications between a central control station and a fleet of cars, trucks, or other land based vehicles. Typical users of land mobile radio systems include police departments, fire departments, taxi services and delivery services. A land mobile system can be configured for communications between the control station and all vehicles in a fleet, between the control station and selected vehicles in the fleet, or between different individual vehicles within a fleet.
Conventional land mobile radio systems are typically organized with a number of vehicles and a control station assigned to a single channel. A user assigned to the common channel must wait until no one else is transmitting on the channel before sending a message, since only one transmission at a time can be processed by a single channel. Even when a transmission is addressed to only one user in a conventional system (and therefore not heard by the other users), the other users in the system must wait until the transmission is completed before they can use the system to communicate.
Trunked land mobile radio systems are an improvement over conventional land mobile radio systems in that trunked systems enable two or more users of the system to transmit at the same time. For instance, the control station in a trunked system can be communicating with one of the vehicles in the fleet, while, at the same time, two other vehicles in the fleet can be using the same trunked system to communicate with each other. Trunked systems group a number of channels into a single system, with each channel accessible to each user of the system. Switching protocol automatically finds and engages an open channel when a user initiates a transmission, avoiding channels that are already in use at the time the transmission is initiated. Since each user will only need to communicate over the system part of the time, and because it is unlikely that all users will need to communicate at precisely the same time, the number of channels assigned to a trunked group can always be less than the number of users assigned to the trunked system.
The United States Federal Communications Commission (FCC) has assigned 600 channels in the 800 MHz band for trunked land mobile use. In particular, mobile transmit frequencies are 806-821 MHz, with the repeater transmit frequencies exactly 45 MHz above the mobile transmit frequencies, or 851-866 MHz. Channel spacing is 25 kHz, with the maximum allowed deviation being +/-5 KHz.
Land mobile radio systems have proven to be an economical and effective means for establishing voice communications between a control station and a fleet of mobile vehicles. The capability of land mobile radio systems to transmit data, however, has heretofore been severely limited by both environmental factors and equipment limitations. The high frequency of land mobile channels, and the mobile nature of vehicle based transceivers, for instance, both contribute to multipath fading of land mobile transmission signals. Multipath fading is particularly hostile to the reliable reception of data signals. Strict limitations on transmitter power levels to prevent interchannel interference exascerbates the fading problem.
Equipment limitations present further problems for accurate and timely transmission of data signals over existing land mobile radio systems. Audio filters in voice radio equipment typically restrict available bandwidth to about 3000 HZ. While it would be desirable to use currently existing land mobile voice transceivers for transmission of data, the built-in bandwidth restrictions set by voice transceiver audio band filters inherently limits bilevel data transmission rates to below 1500 baud. Moreover, preferred trunked radio systems use the subaudio band for transmission of system signaling data, further restricting the bandwidth available for transmitting data information.
Various schemes have been attempted for transmitting data over land mobile radio systems. Frequency shift keying (FSK) modulation techniques have been used to transmit data over land mobile voice transceivers. Use of FSK modulation techniques in a voice transceiver, however, restricts transmission rates to under 1500 baud when encoding the data in preferred, bilevel modulation schemes. Higher transmission rates can be obtained using multilevel FSK modulation techniques, but reliability of multilevel FSK modulation, particularly in the high 800 MHz band, is generally unacceptable. Data can be sent at higher rates than that available through FSK modulation by use of dedicated data transceivers. Using dedicated data transceivers, however, increases initial cost of a system, as well as increasing maintenance costs. Furthermore, dedicated data transceivers require assignment of a dedicated data transmission channel, and are therefore incompatible with trunked systems.
A data interface that achieved high speed, reliable data transmission in a land mobile, trunked voice radio system, would be a decided advantage.